In recent years, there has been a growing trend to move away from printing information on paper and instead, to the use of digital documents, which contain digital content. Examples of digital documents include, for instance, portable document format (pdf) documents, electronic spreadsheets, electronic drawings, documents generated through use of a word processing application, and html pages. The digital documents may also include composite documents that include a mixture of different types of formats.
With paper documents, an individual's signature or other handwritten marks are used to determine whether various changes or additions to the documents were made by an authorized individual. However, such modifications are not possible or are inconvenient with digital documents because these types of modifications will require the individual to print the digital document, sign or otherwise mark the document, and scan the marked document to prove that the modifications were made by the individual. This manner of controlling access to the documents, both paper and electronic, is easily attacked and may easily be counterfeited.
Various techniques have been proposed to prevent or reduce attacks and counterfeiting of digital documents. These techniques typically employ a database, such as, but not limited to filesystem, Sharepoint, RAID devices, etc., upon which the digital documents are stored so that access to the database, or a network to which the database is connected, is controlled by preventing access to the database or network to a user unless that user possesses some secret information, such as, a user identification and password. As such, in order for these security techniques to enforce access control on the digital documents, users are required to be granted access to the database or network. However, access to the database or network is typically prohibited or undesirable for various security reasons, such as, to prevent access by contractors or non-employees of the corporate or government entity that owns the database or network. In addition, it is desirable for users to be granted access to the database or network securely.
Another technique for protecting the contents of digital documents is through encryption of the digital documents using various encryption and signature keys, such as in public-private cryptography. Although this form of protection has been found to be relatively effective, there are a number of issues pertaining to efficiently and securely distributing keys to enable a plurality of users to decrypt and verify the signatures of the encrypted digital documents.